Plants grown at farmlands or ordinary home gardens are always exposed to various biological or non-biological stresses. In general, agricultural crops subjected to breed improvement tend to be less resistant to these stresses. In order to reduce biological stress such as agricultural pests and weeds to maintain a crop yield, agricultural chemicals are used such as fungicides, insecticides and herbicides. However, agricultural chemicals may have insufficient effects, and may cause phytotoxicity when improperly used, and may allow agricultural pests and weeds to develop resistance to the agricultural chemicals, and may pose concerns about safety for environmental life. Meanwhile, the right plant in the right place, breed improvement, irrigation, greenhouse, soil improvement and the like are utilized to respond environmental stress such as temperature, moisture, illuminance, soil pH and salt concentration. Attempts have been made for conferring stress resistance using a plant growth regulator and the like, but effects have been unsatisfactory. Further, plant viral diseases may cause serious damage to key crops such as cereal crops, vegetables and fruit trees. However, to date, agricultural chemicals have not been found which sufficiently demonstrate practical effects against plant viral diseases.
Meanwhile, Non-patent Literature 1 describes that ascorbic acid is involved in disease resistance, hormone actions and the like, and Non-patent Literature 2 describes that ascorbic acid affects plant aging. However, even when ascorbic acid is externally given to a plant, its physiological effect is very limited because ascorbic acid is present at a high concentration in a plant body. Therefore, there will be almost no practical effect.
Nonetheless, Patent Literature 1 describes that a certain derivative of ascorbic acid demonstrates a preventive and curative effect against a plant virus disease, and proposes to apply it to a plant. Further, Patent Literature 2 discloses a composition comprising an antimicrobic antibiotic such as neomycin sulfate, and ascorbic acid, and states that this composition can control a plant disease. Moreover, for purposes such as the stabilization of an agrochemically active ingredient (Patent Literature 3) and controlled release (Patent Literature 4), examples have been proposed in which ascorbic acid is used in combination with an agricultural chemical.